Scientists have uncovered a 'previously unappreciated' mechanism which may explain why eating dietary fibre, in particular fermentable inulin, prevents the negative health effects of a high fat diet.

A modern diet of processed foods that are high in fat and low in fibre is linked to a reduced gut microbiota, obesity and metabolic syndrome.

Metabolic syndrome is the name given to a host of related health conditions including raised blood sugar levels, excess abdominal fat, high blood pressure, and blood fat disorders such as low levels of HDL cholesterol and high levels of LDL cholesterol.

In this study, the US researchers uncovered a new mechanism to explain why, in mice at least, adding the fermentable fibre inulin to food can restore the gut microbiota, thus protecting the mice against metabolic syndrome.

Classed as a soluble dietary fibre, inulin is a naturally occurring polysaccharide present in plants.

It can be found in bananas, onion, garlic and artichoke but most commercially-produced inulin comes from chicory.

It is sometimes classed as a prebiotic - food for probiotic bacteria in the gut microbiome.

Contrary to previous studies, which pointed to role for inulin-derived short-chain fatty acids (SCFAs) in reducing inflammation, the team of researchers led by Dr. Andrew Gewirtz, professor in the Institute for Biomedical Sciences at Georgia State University, found that inulin may not require SCFAs.

Instead, inulin promoted bacteria, increased the production of intestinal epithelial cells and restored the expression of the protein interleukin-22 (IL-22).

The the induction of IL-22 expression​is what “fortifies the intestine, thus reducing microbiota encroachment and ameliorating metabolic syndrome”.​

"These results contribute to the understanding of the mechanisms that underlie diet-induced obesity and offer insight into how fermentable fibers might promote better health," ​said Gewirtz.

According to the authors, this was a “previously unappreciated means”​ in how fibre is improves colon health.

The finding has importance because, while the health benefits of fermentable inulin are known, it is difficult to eat the amount needed for these benefits because of the “logistical considerations and adverse effects, especially bloating and flatulence”.​

This is why it is important to define the mechanisms by which fermentable fibre protects against metabolic syndrome, they write.

Study​

Over a four-week period, the researchers gave mice either a grain-based diet, a high-fat-low fibre diet or a high-fat-high fibre diet (with either fermentable inulin fibre or insoluble cellulose fibre).

They found that mice on the inulin-enriched diet had levels low of weight gain and a reduction in the size of fat cells. Inulin also “markedly lowered​” cholesterol levels and prevented abnormal blood sugar levels (dysglycemia).

Insoluble cellulose fibre on obesity and dysglycemia were more modest.

The high fat-diet supplemented with inulin also helped restore the gut microbiota, although not to the levels of the mice on the grain-diet.

The study was funded by the US-based National Institute of Diabetes and Digestive and Kidney Diseases and the Crohn's and Colitis Foundation.